What Causes Blood Clots During Menstruation?

im 17 and basically i came on my period the 3rd of february and came of around the 8th, i then came back on the 26th of feb & have pretty much been on since few days before i had my second period around the 25th i was sexually active but full protected. the day after, i came back on, very heavily and thick, so last friday i decided to go to the doctors, he prescribed me with tranexamic adic (500mg) loestrin *20, andferrous sulphate (200mg)

that weekend i had intercoure with my boyfriend, he didnt use protection as i had been using the pill since friday what the doctor prescribed me, after we finished i had bled alot, the blood was very dark. then ever since i had still been bleeding and started getting blood clots. so tuesday i never took them they was causing me bad heaches, eye pains & constapation, that day i didnt really bleed, just lightley, the day after i had of college due to heache, constants goosebumps & swollen glands. i went to the doctor today he done me a urine test for water/bladder infection and it was fine. i really need to know whats up. i dont feel im been given the right information. my periods have always been like this. i never know when i coming on my periods. sometimes i just get a weird feeling in my stomach, my first period usually lasts for like 3-4 days. the second one i have lasts ages, its not even like a period its very dark, thick & has a strong smell. it is very heavy and when i wee i start bleeding aswell. i find loads of little dried bits of blood that come out . i dont know whether this has anything to do with it but i have such a bad memory. i cant even remember what i done yesterday. i sometimes get dizzy, i find it extremley hard to concentrate & am very tired after 12am , & recently my headaches are constant, my eyelids hurt & every so often i get sharp pains down the lower part of my stomach and on the lower sides please help!